Aerodynamically efficient lightweight vertical take-off and landing aircraft with multi-configuration wing tip mounted rotors

ABSTRACT

An aerial vehicle adapted for vertical takeoff and landing using a set of wing tip mounted thrust producing elements for takeoff and landing. An aerial vehicle which is adapted to vertical takeoff with the wings in a horizontal flight attitude then transitions to a horizontal flight path. An aerial vehicle which uses different configurations of its wing tip mounted, VTOL enabling rotors to reduce drag in all flight modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/951,450 to Bevirt et al., filed Jul. 25, 2013, which is herebyincorporated by reference in its entirety.

BACKGROUND

Field of the Invention

This invention relates to powered flight, and more specifically to avertical take-off and landing aircraft with multi-configuration wing tipmounted rotors.

Description of Related Art

There are generally three types of vertical takeoff and landing (VTOL)configurations: wing type configurations having a fuselage withrotatable wings and engines or fixed wings with vectored thrust enginesfor vertical and horizontal translational flight; helicopter typeconfiguration having a fuselage with a rotor mounted above whichprovides lift and thrust; and ducted type configurations having afuselage with a ducted rotor system which provides translational flightas well as vertical takeoff and landing capabilities.

In order to provide efficiency in both vertical take-off and forwardflight modes, improvements to past systems must be made. What is calledfor is a vertical take-off and landing aircraft that incorporatesefficiencies into all use modes.

SUMMARY

An aerial vehicle adapted for vertical takeoff and landing using a setof wing tip mounted thrust producing elements for takeoff and landing.An aerial vehicle which is adapted to vertical takeoff with the wings ina horizontal flight attitude that then transitions to a horizontalflight path. An aerial vehicle which uses different configurations ofits wing tip mounted, VTOL enabling rotors to reduce drag in all flightmodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aerial vehicle in forward flightaccording to some embodiments of the present invention.

FIG. 2 is a front view of an aerial vehicle in a forward flightconfiguration according to some embodiments of the present invention.

FIG. 3 is a perspective view of an aerial vehicle in takeoffconfiguration according to some embodiments of the present invention.

FIG. 4 is a front view of an aerial vehicle in takeoff configurationaccording to some embodiments of the present invention.

FIG. 5 is a perspective view of an aerial vehicle in takeoffconfiguration with some cowlings removed for viewing, according to someembodiments of the present invention.

FIG. 6 is a front view of the wing and wing tip of an aerial vehicle inforward flight configuration according to some embodiments of thepresent invention.

FIG. 7 is a top view of the wing and wing tip of an aerial vehicle inforward flight configuration according to some embodiments of thepresent invention.

FIG. 8 is a top view of the wing and wing tip of an aerial vehicle intakeoff configuration according to some embodiments of the presentinvention.

FIG. 9 is a front view of the wing and wing tip of an aerial vehicle intakeoff configuration according to some embodiments of the presentinvention.

FIG. 10 is view of the wing tip rotor of an aerial vehicle according tosome embodiments of the present invention.

FIG. 11 is a view of the wing tip rotor and blade linkages of an aerialvehicle according to some embodiments of the present invention.

FIG. 12 is a partial internal view of the wing tip rotor and bladelinkages of an aerial vehicle according to some embodiments of thepresent invention.

FIGS. 13A-D illustrate the deployment of the tip rotors from a stowed toa deployed position.

FIG. 14 illustrates an aerial vehicle with multiple wing tip rotorsaccording to some embodiments of the present invention.

FIG. 15 illustrates data relating to vorticity magnitude for designsaccording to some embodiments of the present invention.

FIGS. 16A-B illustrate data relating to pressure and pressure contoursfor designs according to some embodiments of the present invention.

FIG. 17 illustrates pitch rotation of the wing tip rotors according tosome embodiments of the present invention.

DETAILED DESCRIPTION

Although vertical takeoff and landing (VTOL) aircraft have always beendesired, compromises in the realization of these aircraft have limitedtheir usefulness and adoption to certain niches. Notably, helicoptersare relatively loud, slow, short-ranged, and expensive to operate. Thepresent invention is capitalizing on advances in electric motors,battery technology, and control systems to create revolutionary VTOLaircraft that are quiet, safe, and efficient.

In some embodiments of the present invention, as seen in FIGS. 1 and 2,an aerial vehicle 100 is seen in forward flight configuration. Theaircraft body 101 supports a left wing 102 and a right wing 103. Theaircraft body 101 extends rearward is also attached to raised horizontalstabilizer 104 which may be attached to a vertical stabilizer with arotational coupling. The horizontal stabilizer has a rear motor 105attached thereto.

The right wing 103 has wing tip features 106, 107 adapted to providelift and reduce drag. The left wing 102 has wing tip features 108, 109also adapted to provide lift and reduce drag. In a typical forwardflight operating scenario, the aerial vehicle 100 may fly as atraditional airplane, although powered by a rear mounted motor andpropeller 105.

FIGS. 3 and 4 illustrate a perspective view and a front view,respectively, of the aerial vehicle 100 in a takeoff (or landing)configuration. In this configuration, using the right wing 103 as anexample, the wing tip features 106, 107 have been reconfigured relativeto each other, and relative to the right wing 103. In this reconfiguredconfiguration, the wing tip features 106, 107 are able to operate as avertically oriented propeller, powered by an electric motor 110 mountedwithin the right wing 103. The raised rear elevator 104 is seen in FIGS.3 and 4 in a takeoff configuration where the rear elevator has rotatedrelative to the vertical stabilizer, and to the aircraft body 101, toallow the rear motor 105 to provide a predominantly vertical thrust. Theaircraft's center of gravity is located between the wing and the tail,so when tilted upwards, this propeller of the rear motor 105 providesadequate pitch control in both directions in vertical flight viadifferential RPM control of the tail propeller and the wingtip rotors.This propeller tilts forward during transition and is the solepropulsion source in horizontal flight. The horizontal tail, placed in aT-tail configuration, tilts with the propeller to reduce download on thesurface and provide additional control by always locating the elevatorin the propwash. Placing this propeller on the tail instead of on thenose provides the benefit of reduced scrubbing drag in horizontalflight. The vertical tail is swept such that the leading edge isvertical to maximize propeller clearance.

In steady vertical flight, the three rotors are nominally run at a lowtip speed of 350 feet/second to significantly reduce noise duringtakeoff and landing. Custom electric motor designs eliminate the needfor gearboxes, reducing weight and noise and improving reliability. Anactive control system stabilizes the aircraft in vertical andtransitional flight, reducing pilot workload and simplifying control.Conventional takeoffs and landings are possible in the horizontal flightconfiguration.

Of note is the design of the wing tip features and their use in aforward flight configuration, as seen in FIGS. 6 and 7, and their use ina takeoff configuration, as seen in FIGS. 8 and 9. A two-bladedvariation has been designed in which both blades pivot to become twoseparate tandem wingtips in horizontal flight. Employing two bladesimproves rotor efficiency and reduces cyclic loading. This geometryresults in the airflow arriving from the leading edges of the blades inboth vertical and horizontal flight, requiring fewer design compromises,and better efficiency in all flight modes. The two blades balance thepropeller, precluding the need for a counterweight.

As seen in FIGS. 10 and 11, a brushless electric motor is located ineach wingtip. The stator is fixed to or within the wing, and the bladesare rigidly attached to the rotor, without flapping, lead/lag, orfeathering hinges. In vertical flight, the blades are locked 180 degreesaway from each other to form a conventional two-bladed rotor, and inhorizontal flight, the blades are repositioned to act as two discretetandem wingtips. Dihedral in the wing provides adequate clearancebetween the wing and the blades in vertical flight. Roll control invertical flight is provided by differential RPM control of the twowingtip motors.

FIG. 12 illustrates an aspect of the multi-configuration tip rotorsaccording to some embodiments of the present invention. An electricmotor 110 embedded in wing, such as right wing 103, would have itsstator 112 embedded into, or otherwise affixed to, the wing structure.The rotor 111 of the motor 110 is adapted to rotate within the stator112 around a motor rotation axis 120. The wing tip features 106, 107which also become blades as a propeller for the motor in the takeoffconfiguration, deploy from their individual first locations relative toeach other, and to the rotor, to their individual second locationsrelative to each other, and to the rotor. Within the rotor 111, each tiprotor may be able to rotate around its own deployment axis 121, 122. Aseach tip rotor rotates around its deployment axis to its individualdeployed position, the tip rotors seen as vortex shedding wing tipfeatures rotate, and reconfigure, to the deployed position of a twobladed propeller. Again, the tip rotors as propeller blades use aconfigured position well adapted for propeller flight, and the tiprotors as wing tip features use a position well adapted for low drag andvortex shedding. In both configurations, the leading edges into theapparent wind are the designed leading edges of the tip rotors.

The transition of the wing tips to rotor blades may include thefollowing sequence: 1) A solenoid releases, unlocking the blades. Thissolenoid holds the blades in position during cruise. 2) The rotor (B)rotates 80 degrees; during this rotation, linkages rotate the bladesaway from each other into diametrically-opposed positions. 3) A secondsolenoid releases the linkage plate inside the rotor, and the blades arenow held in position fixed relative to each other by a spring. In thisconfiguration, rotation of the rotor causes both blades to rotate in thesame direction.

FIG. 13A-D illustrate the transition of the tip rotors from a forwardflight configuration as tandem wing tips to a takeoff position as motordriven propellers. As seen, the tip rotors are configured as tandem wingtips in the forward flight configuration of FIG. 13A. The tip rotorstransition to a takeoff configuration, until that configuration isreached, as seen in FIG. 13D. In some embodiments, the wing tip rotorsare attached to the wings with a controllable rotary mechanism such thatthe rotors may be adjusted in pitch while in use during takeoff andlanding. In some aspects, the rotation of the wing tip rotors in thepitch axis will facilitate transition from vertical to horizontal flightmodes. FIG. 17 illustrates the tip rotors in a pitched position.

Design and Analysis: Initial configuration designs were performed usinga purpose-written configuration analysis code. This code allowed theparametric definition of a configuration and mission; using thisdefinition, the component masses and moments of inertia were estimated,and AVL, a vortex-lattice tool that was developed at the MassachusettsInstitute of Technology, was run to estimate drag and stability of thewings and tails. Statistical methods were employed to estimate theeffects on drag and static longitudinal stability of the fuselage, aswell as other parasitic drag sources not properly captured byvortex-lattice analysis (interference drag and leakage and protuberancedrag). The code began with a provided takeoff weight and computed theavailable payload mass, allowing range to be computed if a portion ofthis payload mass is used for fuel or batteries. Although thisconfiguration ties the disk loading and planform geometry together inunusual and unfamiliar ways, use of this code in this way aids in theidentification of important trends and tradeoffs. For example, if thewingtip blade radius is too high for a given total wing area and aspectratio, the wing taper ratio becomes too high to be structurallyefficient, resulting in a tradeoff in power requirements between cruise(through wing size and aspect ratio) and vertical flight (through diskloading).

Initial aerodynamic and acoustic design of the rotor blades wasperformed using a blade-element momentum design and analysis code suiteemploying the 2D viscous panel code XFOIL to estimate sectionaerodynamics, Goldstein's vortex theory to predict induced velocities,and the Ffowcs Williams-Hawkings equation to estimate acoustics. Due toincreasing dynamic pressure with radius when the blades are operated asrotor blades, rotor performance is more sensitive to the design of theouter portion of the blades; conversely, due to the larger chord nearthe root, performance in horizontal flight, when the blades arepositioned as wingtips, is more sensitive to the root design. Therefore,twist and chord in the inboard portion of the blades were chosen toimprove aerodynamics in horizontal flight by imposing constraints in therotor design code. Airfoils were chosen by 2D viscous panel codeanalysis of many airfoil designs, including custom designs, at theconditions encountered in both the vertical flight configuration, whenthe blades act as rotor blades, and the forward flight configuration,when the blades act as wingtips. Blade thickness was conservativelychosen to preclude the possibility of adverse aeroelastic effects.

Extensive CFD analyses have been performed to further guide design ofboth the airframe configuration and wingtip blades. STAR-CCM+, acommercial CFD code, was used for both mesh generation and CFDsolutions. Unstructured meshes were employed, and Navier-Stokessimulations were run using the SST k-ω turbulence model and the γ-Reθtransition model. These simulations were used to tune the spanwise liftdistribution of the aircraft in forward flight for maximum efficiency,check stability predictions of lower-fidelity analyses, and estimaterotor efficiency and download caused by the wing. A CFD result for a55-pound prototype at 43 knot cruise at standard sea level conditions isshown in FIG. 15, showing the complex tip vortex system that develops.Results of this configuration show a L/D in excess of 20 at this flightcondition, although this is expected to be somewhat reduced in theflight vehicle due to control surface gaps, antennas, motor cooling,etc.

FIGS. 16A-B shows results of CFD analyses of the blade performance of a55-pound prototype in a 100 ft/min axial climb at standard sea levelconditions. Uninstalled figure of merit (computed as the ratio of idealclimb power required from momentum theory to computed power) is 72%, andthe required thrust increment due to the download on the wing is 13%.

FIG. 16A illustrates uninstalled blades with isosurfaces of constantvorticity magnitude and with blades contoured by Cp* (the pressureco-efficient non-dimensionalized by the local rotational velocity). FIG.16B illustrates The blades and the wing, with 25% chord flaps deflectedto 55 degrees, showing download effects on the wing via pressurecontours.

FIG. 14 illustrates another embodiment of an aerial vehicle using tiprotors on both the wings and the rear elevators.

As evident from the above description, a wide variety of embodiments maybe configured from the description given herein and additionaladvantages and modifications will readily occur to those skilled in theart. The invention in its broader aspects is, therefore, not limited tothe specific details and illustrative examples shown and described.Accordingly, departures from such details may be made without departingfrom the spirit or scope of the applicant's general invention.

What is claimed is:
 1. An aerial vehicle adapted for vertical takeoffand horizontal flight, said aerial vehicle comprising: an aerial vehiclebody; a flight control system, said flight control system adapted tocontrol the attitude of said aerial vehicle while taking off verticallyby varying the thrust of the three or more thrust producing elements; afirst left wing, said first left wing comprising a first left wing tiprotor; and a first right wing, said first right wing comprising a firstright wing tip rotor, wherein said wing tip rotors are powered byelectric motors mounted at the tips of said wings, said electric motorscomprising a rotor and a stator, and wherein each of said wing tiprotors comprises a first blade and a second blade, and wherein saidfirst left wing tip rotor and said first right wing tip rotor areadapted to deploy from a forward flight configuration wherein said wingtip rotors are configured as tandem wingtips to a vertical takeoffconfiguration wherein said wing tip rotors are configured as a twobladed propeller, and wherein said first blade and said second bladehave leading edges during vertical flight, and wherein each of saidleading edges of said first blade and said second blade face forwardwhen in said forward flight configuration, and wherein said electricmotors rotate around a first axis, and wherein said first blade deploysfrom a forward flight configuration to a vertical takeoff configurationby rotating around a second axis within the rotor.
 2. The aerial vehicleof claim 1 wherein said second blade deploys from a forward flightconfiguration to a vertical takeoff configuration by rotating around athird axis within the rotor.
 3. The aerial vehicle of claim 1 furthercomprising: a vertical stabilizer attached to said vehicle body; and amotor attached to said vertical stabilizer, said motor adapted to rotatefrom a forward facing configuration providing rear ward thrust to avertical facing configuration providing downward thrust.
 4. The aerialvehicle of claim 2 further comprising: a vertical stabilizer attached tosaid vehicle body; and a motor attached to said vertical stabilizer,said motor adapted to rotate from a forward facing configurationproviding rear ward thrust to a vertical facing configuration providingdownward thrust.